For love, I endure even to God, you must not conquer. He gave animals to man, to which the heart is being ruled (by).

"You are not to be conquered by love, [who is] certainly a harsh god. For Nature gave man a soul by which his heart is governed."

"amori, duro quidem deo" is the dative of agent with the gerundive "es vincendus." "duro" is the adjective "durus," not a verb. You seem to have let out "natura" in the second sentence. "Animum" is from "animus," which means soul/spirit, not "animal," which would be "animal, animalis, n." "quo" is an ablative of means, referring to the animus, "by which."